fix(windows): reap orphaned MCP processes when their parent exits (#692, #576, #680) (#711)

On Windows the PPID watchdog could never fire: orphans aren't reparented, so
`process.ppid` stays constant after the parent dies (defeating the ppid-change
check), and the standalone bundle pre-bakes `--liftoff-only`, skipping the
relaunch that sets `CODEGRAPH_HOST_PPID` (defeating the host-liveness check).
With neither signal available, an orphaned proxy / direct server ran forever,
the shared daemon never saw the client disconnect, and its idle timer never
armed — node processes accumulated until CPU saturated.

Add a win32-only signal: poll the original parent's liveness directly, since
ppid is stable there. Gated to Windows so POSIX double-fork cases keep relying
on the ppid-change signal (a dead original parent is not proof of orphaning on
POSIX). The decision is extracted into a pure, unit-tested helper shared by all
three watchdog sites (proxy socket, proxy local-handshake, direct mode).

Validated on a real Windows 11 VM: in the exact bundle scenario (direct mode,
no HOST_PPID) an orphaned server now exits within one watchdog poll via the new
path; the POSIX reparent path is unchanged and its integration test still passes.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/mcp/index.ts

@@ -49,6 +49,7 @@ import {
 } from './daemon';
 import { connectWithHello, runLocalHandshakeProxy } from './proxy';
 import { getDaemonSocketPath } from './daemon-paths';
+import { supervisionLostReason } from './ppid-watchdog';
 import { HOST_PPID_ENV } from '../extraction/wasm-runtime-flags';
 
 /**
@@ -423,13 +424,13 @@ export class MCPServer {
     const pollMs = parsePpidPollMs(process.env.CODEGRAPH_PPID_POLL_MS);
     if (pollMs <= 0) return;
     this.ppidWatchdog = setInterval(() => {
-      const current = process.ppid;
-      const ppidChanged = current !== this.originalPpid;
-      const hostGone = this.hostPpid !== null && !isProcessAlive(this.hostPpid);
-      if (ppidChanged || hostGone) {
-        const reason = ppidChanged
-          ? `ppid ${this.originalPpid} -> ${current}`
-          : `host pid ${this.hostPpid} exited`;
+      const reason = supervisionLostReason({
+        originalPpid: this.originalPpid,
+        currentPpid: process.ppid,
+        hostPpid: this.hostPpid,
+        isAlive: isProcessAlive,
+      });
+      if (reason) {
         process.stderr.write(
           `[CodeGraph MCP] Parent process exited (${reason}); shutting down.\n`
         );